1. Field of the Invention
The invention concerns a method for generating magnetic resonance spectroscopy data from a volume of interest in the body of a person to be examined with as short an echo time as possible as well as a magnetic resonance system and a digital data storage medium encoded with programming instructions, to implement such a method.
2. Description of the Prior Art
The inventive method is applied in particular to the magnetic resonance spectroscopy of the liver. Diseases of the liver are often associated with an increased fat content in the liver, e.g. liver diseases as a result of obesity. The determination of the fat content in the liver is of great interest as a detail in the diagnosis. 1H-magnetic resonance spectroscopy (MRS) provides a means of determining the fat content of the liver in a non-invasive manner and may be very sensitive to small quantities or smaller changes in the hepatic fat content.
One problem with determining the fat content in the liver by means of 1H-MRS is the variability in the T2 relaxation time. Single-voxel spectroscopy (SVS) normally requires sequences which excite a volume of interest (VOI) or voxels within the liver by means of three slice-selective pulses and acquire a signal from this volume, a relatively long echo time (TE). In the case of a single-voxel STimulated Echo Acquisition mode (STEAM) sequence, three consecutive 90° pulses are used for instance and a stimulated echo is recorded thereafter. The shortest possible echo time in this method is however in the range above 10 ms If patients differ significantly in terms of T2 relaxation time, for instance due to different iron contents in the liver, this results in significant errors in the determination of the relative fat content in the liver.
In the Article by Pineda, P. Sharma, Q. Xu, X. Hu, M. Vos and D. Martin “Measurement of Hepatic Lipid: High-Speed T2-Corrected Multiecho Acquisition at 1H MR Spectroscopy—A Rapid and Accurate Technique”, Radiology: Vol. 252: No. 2 (August 2009), the STEAM sequence is therefore repeated with different echo times and an exponential decay curve is fitted into the acquired spectra on the integral below the fat and water signal, said decay curve forming the T2 relaxation. The signal can thus be extrapolated at time instant TE=0 and the T2 effect can thus be calculated therefrom. This method nevertheless requires the multiple repetition of the spectroscopy measurement, wherein the results are not added so as to increase the signal-to-noise (SNR) ratio but are used again to determine a further parameter (T2). Relatively long acquisition times and large VOIs are therefore offered using this technique.